The present invention relates to optical fibers. More particularly, it relates to a method for manufacturing a single-mode optical fiber in which the core is formed of lamina, adjacent ones of which have different characteristics.
In certain applications of single-mode optical waveguide fibers, e.g. gyroscopes, sensors and the like, it is important that the propagating optical signal retain the polarization characteristics of the input light in the presence of external depolarizing perturbations. This requires the fiber to have an azimuthal asymmetry of the refractive index profile. Copending Application Ser. No. 499,898 entitled "Polarization Retaining Single-Mode Optical Waveguide" filed in the name of V. A. Bhagavatula et al. on even date herewith describes a fiber, the core of which is formed of a plurality of laminae of transparent material, adjacent ones of which have different refractive indices. It would be advantageous to form the core of such a fiber by a chemical vapor deposition (CVD) technique so that the fiber is suitable for transmitting optical signals over long distances.
Methods of forming optical fibers having an azimuthal asymmetry of the refractive index profile are disclosed in U.S. Pat. Nos. 3,980,459 (Li) and 3,982,916 (Miller). In accordance with the teachings of both the Li and Miller patents, one or more layers may be formed on the inner surface of a substrate tube by flowing a reactant vapor through the substrate tube and heating the substrate tube and contained vapor mixture with a moving heat source, external to the substrate tube, such that a suspension of glass particles is produced within the substrate tube, the particles traveling downstream and being fused to form a continuous glassy deposit on the inner surface.
In accordance with the teachings of the Li patent at least one longitudinally-extending rod is inserted into the substrate tube, and the vapor-flowing and substrate-heating steps are restarted to resume the production of a glassy deposit, the deposit forming on the glassy deposit previously formed and on the rod. Finally, the vapor-flowing and substrate-heating steps are terminated when the glassy deposit has attained the desired thickness. The resultant preform can be collapsed and drawn into an optical fiber having an azimuthal asymmetry in its refractive index profile. This method is disadvantageous in that it employs as part of the light-propagating portion of the fiber longitudinally extending regions formed by the inserted rods. The boundaries between these longitudinal regions and the remainder of the core are more likely to be the source of imperfections such as seeds than would be the case if the entire core had been formed by chemical vapor deposition within the substrate tube.
The Li patent teaches a further embodiment wherein the longitudinal strip is employed as a baffle to prevent the buildup of one of the core layers. The baffle is removed and the preform is collapsed and drawn into a fiber. This embodiment can also result in imperfections in the core due to damage caused by removing the baffle. Furthermore, the method of forming a preform is made inordinately complicated by rotating a baffle along with the tube during the deposition process and by the need to remove the baffle prior to the collapse step.
The method of the Miller patent departs from the conventional CVD process in that it includes the step of asymmetrically heating the substrate tube to preferentially deposit the doped particulate material on at least one region of the inner surface thereof. The heating of the hollow substrate tube is deliberately made asymmetrical by employing a plurality of oxy-hydrogen jets on one side of the substrate tube and a plurality of cooling jets on the other side thereof. It is stated that the substrate tube is advantageously rotated during the deposition process to minimize deformation of the substrate tube and that both the cooling and the heating jets must be rotated along with the substrate tube. A rather complicated apparatus is required to rotate the air jets and the gas jets in synchronism with the rotation of the substrate tube.
It is therefore an object of the present invention to provide a method forming an optical fiber preform having a core region comprising laminations of glass, adjacent ones of which have different physical characteristics.